Vehicle systems may include various vacuum consumption devices that are actuated using vacuum. These may include, for example, a brake booster. Vacuum used by these devices may be provided by a dedicated vacuum pump. In still other embodiments, one or more ejectors may be coupled in the engine system that may harness engine airflow and use it to generate vacuum.
The inventors herein have recognized that ejector configurations may cease to provide increasing vacuum with decreasing outlet pressure. The inventors have further discovered that ejectors can continue to provide increasing vacuum with decreasing outlet pressure if the pressure at the motive inlet port is reduced. Thus, by throttling the motive air flow rate through an ejector in such a way to maintain the pressure ratio of the ejector (that is, the ejector outlet pressure relative to the ejector motive flow inlet pressure) at or above a threshold ratio (e.g., at or above 0.71), an existing ejector may continue to produce deeper ultimate vacuums even as the source vacuum is reduced.
In one example, the above issue may be at least partly addressed by a method for an engine comprising: closing a throttle upstream of an ejector coupled to an intake manifold to increase level of vacuum generation by the ejector during a first mode of operation; and opening the throttle to increase rate of vacuum generation by the ejector during a second mode of operation. In this way, a fast vacuum pump-down is achieved at lower manifold vacuums and a deeper yet vacuum is achieved at the ejector at higher manifold vacuums.
As an example, an engine system may include an ejector coupled to an intake manifold in a conduit coupled upstream of a charge air cooler, the conduit in parallel to an air intake passage. A first ejector throttle may be coupled immediately upstream of the ejector, without other devices or flow couplings there-between, for enabling pressure reduction at the ejector. A second air intake throttle may be coupled to the intake manifold, downstream of the charge air cooler, in the air intake passage. During conditions when intake manifold vacuum is lower, at least a portion of intake air may flow through the ejector in the conduit with the first throttle open a first amount to generate vacuum for an engine vacuum consumption device (such as a brake booster). By flowing air through the ejector with the first throttle more open, the high suction flow rate or pumping flow rate through the ejector can be advantageously used to rapidly raise a vacuum level of the vacuum consumption device. However, the ultimate vacuum level attained may not be deep enough, for example, the level attained may be lower than a desired vacuum level. When intake manifold vacuum is higher (such as during low load conditions), the desired vacuum level may be attained by flowing air through the ejector with the first throttle open a second amount that is more closed than the first amount. By flowing air through the ejector with the first throttle more closed, a pressure upstream of the ejector can be lowered to raise the ultimate vacuum level to the desired vacuum level, albeit at a lower pumping flow rate. Adjustments to the first throttle may be compensated for by corresponding adjustments to the second throttle to maintain air flow to the intake manifold. Thus, when an opening of the first throttle is increased, an opening of the second throttle may be correspondingly decreased, and vice versa.
In this way, each of a high vacuum pumping rate and a deeper ultimate vacuum can be achieved using an existing engine system ejector. By opening a throttle immediately upstream of the ejector to raise an upstream pressure, a rapid vacuum pump-down can be achieved during lower manifold vacuums. Then, by closing the throttle upstream of the ejector to lower the upstream pressure, a deeper yet vacuum level can be achieved during higher manifold vacuums at a slower pumping rate. In one example, the deeper vacuum may be advantageously used to provide vacuum to a brake booster for a single brake stop at a high g-force (e.g., a “panic stop”). Overall, a vacuum generation efficiency of the ejector is increased without substantially increasing component cost or complexity.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.